Re: [PATCH 21/24] rcu/tiny: move kvfree_call_rcu() out of header

Uladzislau Rezki
2020-05-06T18:29:11Z

Hello, Paul, Joel.

> > Move inlined kvfree_call_rcu() function out of the
> > header file. This step is a preparation for head-less
> > support.
> >
> > Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org>
> > Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
> > ---
> > include/linux/rcutiny.h | 6 +-----
> > kernel/rcu/tiny.c | 6 ++++++
> > 2 files changed, 7 insertions(+), 5 deletions(-)
> >
> > diff --git a/include/linux/rcutiny.h b/include/linux/rcutiny.h
> > index 0c6315c4a0fe..7eb66909ae1b 100644
> > --- a/include/linux/rcutiny.h
> > +++ b/include/linux/rcutiny.h
> > @@ -34,11 +34,7 @@ static inline void synchronize_rcu_expedited(void)
> > synchronize_rcu();
> > }
> >
> > -static inline void kvfree_call_rcu(struct rcu_head *head, rcu_callback_t func)
> > -{
> > - call_rcu(head, func);
> > -}
> > -
> > +void kvfree_call_rcu(struct rcu_head *head, rcu_callback_t func);
> > void rcu_qs(void);
> >
> > static inline void rcu_softirq_qs(void)
> > diff --git a/kernel/rcu/tiny.c b/kernel/rcu/tiny.c
> > index aa897c3f2e92..508c82faa45c 100644
> > --- a/kernel/rcu/tiny.c
> > +++ b/kernel/rcu/tiny.c
> > @@ -177,6 +177,12 @@ void call_rcu(struct rcu_head *head, rcu_callback_t func)
> > }
> > EXPORT_SYMBOL_GPL(call_rcu);
> >
> > +void kvfree_call_rcu(struct rcu_head *head, rcu_callback_t func)
> > +{
> > + call_rcu(head, func);
> > +}
> > +EXPORT_SYMBOL_GPL(kvfree_call_rcu);
>
> This increases the size of Tiny RCU. Plus in Tiny RCU, the overhead of
> synchronize_rcu() is exactly zero. So why not make the single-argument
> kvfree_call_rcu() just unconditionally do synchronize_rcu() followed by
> kvfree() or whatever? That should go just fine into the header file.
>
Seems it does not go well if i do it in header file:

<snip>
diff --git a/include/linux/rcutiny.h b/include/linux/rcutiny.h
index 0c6315c4a0fe..76b7ad053218 100644
--- a/include/linux/rcutiny.h
+++ b/include/linux/rcutiny.h
@@ -13,6 +13,7 @@
#define __LINUX_TINY_H

#include <asm/param.h> /* for HZ */
+#include <linux/mm.h>

/* Never flag non-existent other CPUs! */
static inline bool rcu_eqs_special_set(int cpu) { return false; }
@@ -36,7 +37,15 @@ static inline void synchronize_rcu_expedited(void)

static inline void kvfree_call_rcu(struct rcu_head *head, rcu_callback_t func)
{
- call_rcu(head, func);
+ if (head) {
+ call_rcu(head, func);
+ return;
+ }
+
+ // kvfree_rcu(one_arg) call.
+ might_sleep();
+ synchronize_rcu();
+ kvfree((void *) func);
}
<snip>

kvfree() is defined in <linux/mm.h> as extern void kvfree(const void *addr);
If i just include <linux/mm.h> i get many errors related to "implicit declaration
of function" like:

<snip>
rcu_read_lock()
compound_mapcount_ptr()
rcu_assign_pointer()
...
<snip>

and many other messages like:

<snip>
warning: returning ‘int’ from a function with return type
error: unknown type name ‘vm_fault_t’; did you mean ‘pmdval_t’?
error: implicit declaration of function ‘RB_EMPTY_ROOT’
...
<snip>

Please see full log here: ftp://vps418301.ovh.net/incoming/include_mm_h_output.txt

I can fix it by adding the kvfree() declaration to the rcutiny.h also:
extern void kvfree(const void *addr);

what seems wired to me? Also it can be fixed if i move it to the tiny.c
so it will be aligned with the way how it is done for tree-RCU.

Any valuable proposals?

--
Vlad Rezki